Since 1992 Global Mean Sea level has been rising at a fairly linear rate of 3.2mm/year as measured and calculated by the University of Colorado .

Approximately half of this is due to ice melt and water pumped from aquifers. The remainder is due to thermal expansion. If this rate remains constant the sea level rise over the coming century would be 320mm. I intend to discuss the scientific evidence for processes likely to accelerate this rate and produce sea level changes greater than 1 metre over the same period.

The extra volume will come from three main sources. I will discuss changes in the ice melt from Antarctica and Greenland and the possibility of increased rates of thermal expansion.

Since the last glacial maximum sea level has risen by 120 metres in 20,000 years . Why should a rise of 1 metre be a problem? I also intend to discuss the vulnerability of our civilisation to even geologically minor changes.

There must be some pictorial proof showing any of the many shipping ports around the world disappearing under water - even slightly - to support such a claim? All the ports I've frequented in my life (merchant navy) are still as I found them even 30 years on - right through the 'massive' temperature rises we're having.

The orbital changes observed would indicate either flat conditions or a reduction in 65N inaolation. This should be producing cooling, glacier and ice sheet growth and lower sea level. Instead, as I will demonstrate, we are seeing ice melt, sea level rise and warming.

With CO2 40% above normal interglacial levels and climate trends oppoaiopposingg orbital trendstrends, applying normal interglacial behaviour to our current climate is not appropriate. We are in new territory here.

EM: so, let me get this straight; you are saying that ANY rise in sea level is a threat to civilization? Or do you mean a rise of an as-yet unspecified magnitude? What sort of threat is it that you fear, and how will it be revealed?

As the sea level has risen by over 300mm over the last 150 years or so, without any significant threat to civilization, quite why would a similar rise be more of a threat?

You are also presenting your idea based upon one institution’s work; are there other sources, and do you have their examples? (I do know of one, who claims that sea levels are more or less static, but he is considered out-of-date.)

Approximately half of this is due to ice melt and water pumped from aquifers. The remainder is due to thermal expansion.

Is this information in the cited source, or your own suppositions? If the latter, do you have anything to back that up with?

As I have been involved with measuring liquid surface levels in confined spaces on an unstable platform (by many and various means), I am fully aware of the difficulty of getting a measurement accurate to within 5mm within such a space, so get very sceptical – even to the point of suspicious – of more accurate claims over the entire, unconfined ocean surface.

And to the surprise of no-one, Entropic Man is not letting inconvenient facts get in the way of promoting his pet theory.

"What does The Milankovitch Theory say about future climate change?

Orbital changes occur over thousands of years, and the climate system may also take thousands of years to respond to orbital forcing. Theory suggests that the primary driver of ice ages is the total summer radiation received in northern latitude zones where major ice sheets have formed in the past, near 65 degrees north. Past ice ages correlate well to 65N summer insolation (Imbrie 1982). Astronomical calculations show that 65N summer insolation should increase gradually over the next 25,000 years, and that no 65N summer insolation declines sufficient to cause an ice age are expected in the next 50,000 - 100,000 years."

Just as a re-cap: I understand it previous interglacials were warmer than the current one, and we're in a cooler phase of the current one. The question I have is that it is likely that sea levels have been higher in the current interglacial by thermal expansion if nothing else. Secondly previous interglacials must have had higher sea levels than those of today.

Therefore the questions are1 have homosapiens experienced higher sealevels than today?2 has life on Earth experienced higher sealevels than today?

If the answer to either of these questions is yes and we're as good as we think we are then we should be able to survive. There may be problems but many other things could have a more dramatic impact especially those causing a major changes in a short timescale.

I have a feeling this is another of Entropic man's Private Frazer discussions. Both he and Private Frazer may be correct in time especially if every natural variation is a harbinger of doom.

Like Latimer, I find the phrase 'threat to civilisation' problematic as a way of framing a discussion on whether sea level rise is a problem or not. Something doesn't have to be a 'threat to civilisation' to be undesirable - it could still alter ways of life for many people, and possibly even cost lives, and/or be very expensive to deal with, without actually destabilising 'civilisation' as such (whatever that means anyway, as Roger Tolson points out).

For background to this discussion, a useful paper is here - authors include Robert Nicholls (University of Southampton), Jason Lowe (Met Office) and Richard Tol (several affiliations).

They look at two scenarios of SLR by 2100 - 0.5m (well within IPCC likely range) and 2m (outside IPCC likely range, but viewed by some as at least plausible even if unlikely). They estimate the number of people who would be displaced in different regions by these two SLR amounts, if there were no increase in coastal protection, and also the costs of such protection. They say:

[without increased protection] land loss amounts to a total of 877,000–1,789,000km^2 for a 0.5 and 2.0m rise in sea level, respectively (figure 2). This amounts to approximately 0.6–1.2% of the global land area. The net population displaced by this rise is more significant, being estimated at 72 and 187 million people over the century, respectively (roughly 0.9–2.4% of the global population). This reflects the high population density in coastal areas.

The incremental adaptation costs are estimated at roughly between US $25 and $270 billion (1995 values) per annum for 0.5 and 2.0m in 2100, respectively.

It's important to remember that there's a long lag between warming and SLR - whether the sea level response is large or small, it will probably be ongoing for several centuries, so we're committing to an ongoing change of some (largely unknown) rate and magnitude. This paper just looked out to 2100.

It is a matter of opinion whether / which of the above are acceptable / undesirable or not, in comparison with the costs and impacts of mitigation. This would be a useful and interesting debate. May I suggest that for the purposes of a constructive discussion here, it would be useful to take the above numbers at face value and debate their implications?

The Holocene has been going for c. 12000 years. Is NOAA saying that it will last from c. 60000 to 110000 years?

Let's hope they are right.

I like this comment on Wiki:

"The earth has been in an interglacial period known as the Holocene for more than 11,000 years. It was conventional wisdom that the typical interglacial period lasts about 12,000 years, but this has been called into question recently. For example, an article in Nature[35] argues that the current interglacial might be most analogous to a previous interglacial that lasted 28,000 years. Predicted changes in orbital forcing suggest that the next glacial period would begin at least 50,000 years from now, even in absence of human-made global warming[36] (see Milankovitch cycles). Moreover, anthropogenic forcing from increased greenhouse gases might outweigh orbital forcing for as long as intensive use of fossil fuels continues.[37]"

Richard Betts,If slr is not a threat to civilization, then would it be reasonable to review the costs and effectiveness associated with differing strategies to deal with it. The interesting thing in your examples is that neither fit the current slr situation and over egg it quite a bit. We are in the second decade of the 21st. To get 500mm of slr by 2100 means an increase of slr from 3mm per year to about 6mm per year. Starting *now*. Any evidence for that?Additionally ocean and flood defenses are upgraded and replaced/maintained regularly, in well run nations. Are your cost projections on even the lower side based on incremental costs above those already considerable costs, or are they merely the costs that will be required in the normal capital improvements cycles?

Richard Betts I ask this question n the assumption you'll visit more than once or twice.The lag between the end of the Little Iceage and the start of rising sealevels isn't that great according to this data, nor has it changed greatly since the rise started. Global Mean Sea Level Reconstruction since 1700 by Jevrejeva et al, 2008 This is from PMSL who say on their website:-

Established in 1933, the Permanent Service for Mean Sea Level (PSMSL) has been responsible for the collection, publication, analysis and interpretation of sea level data from the global network of tide gauges. It is based in Liverpool at the National Oceanography Centre (NOC), which is a component of the UK Natural Environment Research Council (NERC).

Coastal errosion, a continual process, has been adapted to without any recourse to massive global wealth input/output or disruptive displacement of population. SLR will also be approached (already is being) with the same non-plussed attitude.

Maybe I should be flattered about Adler. But it is not my name. Alder as in the alder tree.

My case is simple. The expected sea level rise within the next 100 years can be handled with today's technologies and is not a 'threat to civilisation'.

Estimates of the rise vary - extrapolation of today's trends measured by our best instruments suggest 9 inches to a foot. Others believe that it might be as much as 3 feet.

But even 3 feet of rise is quite within today's technology to handle. A rise of 3 feet in a tsunami over a period of a minute or two would indeed be a disaster (though whether even such a disaster would be truly 'a threat to civilisation' is debatable) .

But SLR according to the predictions is exactly unlike the sudden and unheralded approach. It is a very very very gradual known and predictable encroachment. We know in general terms that its been going on since the Ice Age and it has been so gradual that for all practical purposes we've never really noticed.

And nowadays, with vastly improved monitoring capabilities, it is still difficult to look back over recent (last 100 year) history and find many practical examples where SLR has made much difference to anything. It's hard to find any places that have been inundated and made uninhabitable. The Maldives have been the poster child for such fears, but even they have so little actual fear of it happening that they are building a new airport for the purpose of importing more tourists...not for evacuating the existing population.

SLR will be such a gradual process that it will be 'no surprises'. A rise of three feet per century is the same depth as a single housebrick (4 inches) every 8 or so years. We have plenty of time to prepare and take action if the more alarmist predictions are seen to be coming true. Or to do less if they don't (as seems currently to be the case).

Richard Betts (above) makes reference to the 'displacement' of between 72 and 185 million people (up to 2.5% of the global population) as a consequence, and up to $270 billion (1995 costs) p.a costs. Big numbers. But not overwhelmingly big. Not catastrophically unimaginably big.

The UK GDP in 1995 was about $1200 billion. So the expected annual costs for the entire globe come out as about the same as the economy of UK produces in about 2 months. Big, unwelcome, inconvenient. But not impossible to handle.

A word on 'displacement'. I'll assume that Tol and colleagues have done a good job in their estimating. 'Displacing 200 million people' sounds like a lot. But we return to the idea that its a gradual foreseeable process, not a sudden one.Very few people born today will expect to live out their entire lives within three feet of the high water mark. They move. They grow up. They move away from their parents. They set up their own homes. Most people voluntarily 'displace' themselves a few times in their lives. Rising SL will mean that some places slowly become uninhabitable. But as people naturally move out, the SL will be just one more pressure on them...rarely an overwhelming catastrophe.

My own city, London. shows a fantastic example of this in action and illustrates how much quicker natural displacement is than any likely SL problem

After WW2 the East End of London and its Docks began a process of slow economic and physical decline. there were many reasons for this - wartime damage, dreadful labour relations, the increasing trend to containerisation.... 35 years ago the population had displaced itself to other places and Docklands (as it was known) was a real wasteland.

But others saw potential for a new financial centre and built a new financial centre on the relic of the old trading lands. In effect the City of London has moved three or four miles in 25 years. Other industries have followed..newspapers (perhaps themselves now a dying industry??) moved away from their historic home in Fleet Street and took the opportunity to change their outdated practices. And Docklands is once more a thriving residential area.

Moral of the story - 'displacement' takes place for all sorts of reasons. The world (and cities especially) are not museum pieces incapable of reacting to change, but almost like autonomic living things. SL will be another catalyst for further change. But it is neither so big nor so overwhelming as to be an outlier among all the other influences of change that already exist.

Final thoughts.

I used to work in Central London right by the River Thames. Thise who have ever watched the famous Oxford vs Cambridge University Boat Race will know that the river is tidal. The traditional start point at Putney was 50 yards from my office window. The tide goes up and down 14 feet every 6 hours and we have a river/sea wall to keep us from getting our feet wet at high tide. I really cannot believe that it will be beyond our capabilities to add a couple more blocks of stone on the top of the wall between now and 2114.

Wile I compose a proper reply you might like to read this , an analysis by Reuters of current sea level behaviour and a couple of personal stories of people whose situation has been affected as a result. Adaptation to even current rates of change would not seem to be easy for those faced with the immediate prospect.